( 703 ) 



curves a negative deflct'lion, becomes in the later ones a positive 

 deflection. The latter, on further lengthening of the duration of liglit, 

 gradually increases in size. In the contlict between negative and 

 positive detlections, there is sometimes seen an upward movement, 

 which is immediately preceded by a small downward one. 



Of the various particularities ^^ hich occur in the course of the 

 experiments we shall only briefly mention the latent period. The 

 duration of this period is dependent to so high a degree upon the 

 intensity of the illumination, that it is possible to some extent to judge 

 of the intensities of light used by previous observers from the latent 

 periods recorded by them. With very weak lighting there appear 

 latent periods of the second substance which may exceed two seconds. 



In opposition to Gotch and Garten Waller ^) also mentions 

 latent periods as large in amount as we have observed and others 

 much lai'ger, but as Waj.ler in his experiments made use of a slow 

 Thomson galvanometer, there remained the possibility that there were 

 two opposite forces which at first neutralised one another and then 

 after an interval one obtained the mastery. The forces assumed by 

 Waller agree with our first and second substances. 



A more detailed description of our experiments accompanied by a 

 reproduction of some of our curves will appear elsewhere. 



Geophysics. — ''The height of the mean sea-level in the Y hef ore 

 Amsterdam from 1700—1860". By Prof. H. G. van dk Sande 

 Bakhuyzen. 



Our section has been engaged in former years with an investigation 

 of the subsidence of the land in the Netherlands, and it is especially 

 to Dr. F. J. Stamkart, member of the committee foi- that investigation, 

 that we owe se\eral important communications on this subject. 



Twenty years ago, when calculating the results of the precise 

 levelling, 1 made some computations in order to determine the 

 subsidence of the land but have not published them. The interesting 

 paper on this subject of Mi'. Ramaer, head-engineer, director of the 

 hydrographic survey, has now induced me to re-examine my former 

 notes and as they perhaps may contribute towards the solution of 

 the problem, whethei' the land under Amsterdam has subsided since 



1) Augustus D. Wallek, l^hilosopli. Transact, of the fioyal Soc. uf London, 

 Ser. B, vol. 193, p. 128, 1900. 



